An electric resistance-type PM sensor is conventionally known as a sensor for detecting PM in exhaust discharged from an internal combustion engine. In the electric resistance-type PM sensor, a pair of conductive electrodes is arranged opposite to each other on a surface of an insulating substrate. A PM amount is estimated based on the fact that the electric resistance value changes with the conductive PM (mainly soot ingredients) adhering to the electrodes (e.g., see Patent Document 1).
A failure diagnostic device of the electric resistance-type PM sensor is proposed, which compares regeneration times of two PM sensors and determines a failure when a difference between the regeneration times of the two PM sensors is greater than a predetermined value (e.g., see Patent Document 2).